The present invention relates to electrostatic discharge protective workstations and particularly the metal components thereby comprising metals cabinets, drawers and shelves; and particularly the method of making the electrostatic discharge protective workstations conform with the EOS/ESD standard for protection of electrostatic discharge susceptible items.
Commonly known as static electricity, electrostatic discharge (ESD) is a transfer of electrons when two objects with dissimilar charges touch, rub, come close together, or are separated. Most objects have dissimilar charges affected by movement, low humidity, and the nature of the material itself. Some of the most common contributors are people, through ordinary activities and synthetic materials such as plastics. Wherever electronic components and spare parts are kept or used, electrostatic discharge damage can occur. The further into the manufacturing process that the damaged component is introduced, the more costly the consequences. As electronic products grow in sophistication, they become more and more sensitive, and are used in increasingly critical and costly applications. As a result, when they fail, the damage can be expensive. One uncontrolled electrostatic discharge can cause partial damage whose effects may not be discovered for months or years or it can cause total destruction, affecting not only the host component, but spreading to others within the system as well. Wherever critical electronic parts are stored and used for today's electronics-driven products and equipment, there is the risk of failure due to electrostatic discharge.
To aid manufacturers to minimize and/or eliminate the effects of electrostatic discharge, the Electrical Over Stress/Electrostatic Discharge (EOS/ESD) Association, Inc. formulated recommended standards to industry for solving electrostatic discharge problems. In 1990, the EOS/ESD Association formulated the standard for worksurfaces, EOS/ESD-S4.1-1990-worksurfaces-resistive characterization. The purpose of an ESD-protective worksurface is to aid in the prevention of damage to ESD-susceptible parts. There are several ways these surfaces may act to provide this protection. One involves the removal of charge residing on the surface of the material. A second charge-removal task involves the charge on an object such as a tote box that is placed on the surface. In this case, the charge must flow across the zone between the object and the worksurface, which can interpose a considerable contact resistance. A third charge removal task involves current flow from a charge-susceptible device placed on the surface. In this case, a low discharge current may be desirable.
The degree of protection afforded by a worksurface is strongly related to the time needed to discharge an object. In practice, some form of resistance value is commonly given as an indication of the effective charge removal characteristics of the worksurface. However, this description is incomplete since discharge time depends on several other factors, such as the effective capacitance of the worksurface, contact resistance and the actual discharge path. (The capacitance does not usually vary as much as resistance.) The other effects are very dependent on the individual situation. As a result, resistance is believed to be the best single predictor of performance of ESD-protective worksurfaces. The aforementioned standard relies on resistive measurements, utilizing standard instruments, to provide a means of evaluating materials or installed worksurfaces.
To provide the convenience of a single location for maintenance and storage of small static-sensitive assembly parts and tools, electrostatic discharge protective equipment was developed specifically for a static-safe work environment. Such equipment included electrostatic workstations comprising worksurfaces, cabinets and drawers, and shelves. It should be noted that the worksurfaces normally are non-metallic materials whereas the cabinets, drawers and shelves may be made of metal. Early electrostatic dissipative cabinets were standard metal cabinets to which the customer attached a ground cord with a one megohm resistor built in. The other end was then connected to a hard ground (water pipe, cooper rod, etc.). This system was used to divert an electrostatic charge from the user's body to ground when he touched the handle of the cabinet. The one megohm resistor protected the user from a high amperage charge if the circuitry was contacted by high electrical energy. However, if the user did not touch the handle, he would by-pass the grounding circuitry and create a potential for damage to anything he touched in the cabinet and/or drawer. Later, plastic manufacturers were spraying their non-conductive plastic components (not working surfaces) with semi-conductive coatings to provide electrostatic discharge protection. It was found that some of these semi-conductive paints when applied to metal cabinets had favorable results at voltages around 10 V DC.
Starting before and during this period, the EOS/ESD Association was being formed as a body to provide specifications and guidance to this new industry. Because of the need for industry to achieve higher voltage protection and also the Department of Defense, the Association began developing test specifications for worksurfaces at both 10 V DC and 100 V DC. The implication of the worksurface testing standard increasing to 100 V DC would have no effect on products made of plastic but would affect products (steel cabinets, etc.) made of steel. The plastic products had a built in dielectric layer and, thus, were not susceptible to effects of any breakdown of the surface coating to the base component at 100 V DC. In the case of metal cabinets, since the electrostatic discharge paint was applied directly to conductive steel, it was recognized that there would be a potential problem when the specifications for components other than the worksurface were approved by the EOS/ESD Association. At voltages up to 10 V DC, the semi-conductive paint on the metal cabinets was satisfactory, however, when the voltage was increased to 100 V DC, the conductive coating broke down and the charge went directly to the base conductive steel thereby defeating the controlled conductive properties of the electrostatic dissipative coating.
After extensive development and testing, there has been developed in accordance with the present invention a method of making an electrostatic workstation where all of the parts thereof conform with the standard in EOS/ESD-S4.1-1990 and Draft EOS/ESD-DS10.1-199X. (The draft standard is still unfinished as of the filing date of this application). This standard is incorporated herein by this reference thereto. The standard requires that electrostatic discharge protective cabinets, accessories and worksurfaces perform within the static dissipative range of point-to-point resistance of 10.sup.6 -10.sup.10 ohms and point-to-ground resistance of 10.sup.6 -10.sup.10 ohms at 10 V DC and 100 V DC so as to provide static dissipation as well as protection against electrostatic overstress on all internal and external static dissipative surfaces.